The present invention relates to a method of riveting which is of particular, but not exclusive, application to the automotive industry.
Self-piercing riveting (SPR) is a spot-joining technique in which a self-piercing rivet is driven, by a punch, into a layered workpiece supported on a die. The die is shaped so that as the rivet is driven into the workpiece towards the die, the material of the workpiece plastically deforms. This flow of workpiece material causes the annular tip of the rivet to flare outwards and remain encapsulated by an upset annulus of the workpiece material. The flared tip of the rivet interlocking with the upset annulus of the workpiece prevents removal of the rivet or separation of the layers of the workpiece.
Because SPR requires plastic flow of workpiece material to allow penetration and upsetting of the rivet, some materials are usually considered unsuitable for this technique. For example, magnesium alloys, ultra high strength steel (UHSS) and aircraft grade aluminium are not considered to have sufficient ductility for conventional SPR—a conventional rivet of sufficient column strength to penetrate materials of this hardness is too resistant to flaring to be properly upset. As another example, polymeric workpiece layers or those of composite materials may crack or fracture upon contact with the rivet, rather than deforming plastically, and this can produce a weak joint and/or one which is more exposed to oxidation through moisture ingress. SPR is therefore conventionally only used for materials such as standard grades of steel and forming grade aluminium.
Solid riveting (i.e. conventional riveting) is another spot joining technique. A rivet with a cylindrical shank and enlarged head is inserted into a pre-formed hole in a workpiece, so that its head abuts the top surface of the workpiece and the shank protrudes from the layered workpiece on the other side. The protruding end of the shank is then upset, for instance using a hammer or press in conjunction with a bucking bar, peening the end of the shank to form a radially enlarged lobe which prevents removal of the rivets or separation of the layers of the workpiece. One problem with solid riveting is the requirement for pre-formed holes in workpieces. This increases the complexity and duration (and thus cost) of the joining process. In addition, steps must be taken to hold a workpiece in position after the holes have been formed, so as to prevent different layers (and thus the holes therein) from becoming misaligned while the rivet and associated tooling is maneuvered into place.
Another known spot-joining technique is friction stir spot welding. In friction stir spot welding a cylindrical punch with a shouldered probe at its tip is rotated and driven into the workpiece layers to be joined. Sliding friction between the probe and the workpiece layers causes the layers to soften and plasticize without melting, and the rotation of the probe displaces the material and causes the plasticized portions of the two layers to intermingle. When the punch is withdrawn and the workpiece allowed to cool, the intermingled plasticized portions harden and produce a welded joint between the two layers.
Friction stir spot welding is only used to weld materials of very similar composition, since the above intermingled plasticized portions can only be formed if the materials of the workpiece soften at similar temperatures. Further, some materials are unsuitable for friction stir spot welding at all, for instance those which do not soften with temperature in the required manner (such as thermosetting polymers), or those which undergo an alteration in mechanical properties at the temperature required (for instance, hardened steel may be brought out of temper in the region of a friction stir spot weld).
It is an object of the present invention to mitigate or obviate one of the aforesaid disadvantages, and/or to provide an improved or alternative method of riveting.